That is a good question , one I've pondered myself . I have asked 4 different alignment shops and got 4 different answers . What I have done is compromise and loaded the truck approximately half way so it wouldn't be off too much either way . It also depends on how many inches the back end squats when loaded , that's what changes the geometry . Another thing to keep in mind is not the amount of time it is loaded but the mileage . It could be loaded 50% of the time but that might only represent 10% of the mileage , or vice versa . Another wild card is if the original specs already take into account it will be fully loaded . Haven't found anyone to answer that yet either .